1. Field of the Invention
The present invention relates generally to vapor recovery systems and more particularly to two stage vapor recovery systems for the separation of solvent vapor from air.
2. Brief Description of the Prior Art
For the purpose of vapor recovery, various methods have been employed to remove the vapor from an air/vapor mixture. One method which has been employed for that purpose is to condense the vapors and to collect the resulting liquid. Another is to drive the air/mixture through an adsorber which preferentially adsorbs the vapor. The adsorbed vapor might then be separated out for reclamation. The combination of condensation and adsorption have also been employed for the purpose of vapor recovery.
Merely performing vapor recovery through condensation by countercurrent flow, although effective, is not without problems. Lowering the temperature of the air/vapor mixture in one stage necessitates rather high refrigeration power requirements. Further, lowering the temperature of the air/vapor mixture in a single stage to below 32.degree. F. creates the additional problem of the formation of ice within the condenser, whether that condenser be a packed column countercurrent flow arrangement, a shell and tube heat exchanger or any other typical condenser arrangement. The formation of water ice will restrict flow through the condenser reducing the efficiency of the condenser and ultimately requiring the condenser to be shut down so that it can be thawed out.
These problems can be significantly reduced by operating a vapor recovery system in two stages, a low temperature stage and a high temperature stage. The high temperature stage would be above the freezing point of water and the low temperature stage would be below the freezing point of water. By taking 60% to 70% of the refrigeration load during the high temperature stage, the refrigeration power requirements can be reduced by approximately 50%. Further, an additional effect of having a first high temperature stage is to remove most of the incoming water contained in the air/vapor mixture. Therefore, there will be less water and consequently, less water ice formed in the low temperature stage reducing the need for shut downs of the vapor recovery systems.